Display rack or shelf structures typically have vertical posts and horizontal rails. The rails often function a stretchers between posts, and may also support shelves. In the typical structure, metal rails are welded or bolted to metal posts. Interlocking tongue and groove connections between the rails and the posts have also been proposed, such as shown in U.S. Pat. Nos. 1,320,982; 2,925,181; 3,045,834; 3,529,857; 3,724,678 and 3,963,290.
In all of these patents (except U.S. Pat. No. 3,529,857), the rails have a flat surface which butts against a flat surface of the post. The surfaces themselves do not interlock and any tendency for the rail to twist with respect to the post must be restrained by other structure. In U.S. Pat. No. 3,529,857, a flattened end of one tube is received in a slotted socket formed in another tube. There are no butted interlocking surfaces.